Ground-based radars receive clutter from many sources. Typical sources of clutter include terrain clutter, vehicle clutter, and volume clutter, by way of example only. Conventional cancellation techniques for cancelling clutter include Doppler processing and non-adaptive or deterministic spatial nulling. Such cancellation techniques, however, introduce into the system undesirable traits and characteristics that limit their utility.
For example, Doppler processing is typically applied to all beam positions that contain clutter, including lower beam positions contaminated by surface and near-surface clutter through their mainlobes and sidelobes, as well as upper beam positions contaminated by surface and near-surface clutter through their sidelobes alone. However, Doppler domain processing introduces potentially significant signal loss, particularly when a broad Doppler null is required to suppress velocity-spread volumetric clutter (e.g. clutter due to birds, chaff, and atmospheric disturbances, known as “angels”). Further, Doppler nulling undesirably generates velocity spans within which the probability of target detection is greatly reduced (otherwise known as blind or dim velocities). In particular, conventional Doppler clutter cancellation in upper beams contaminated by sidelobe clutter introduces velocity blind and dim speeds.
Applying conventional non-adaptive or deterministic spatial nulling to one or more of the transmit or receive waveforms produces a null in the two-way beam's sidelobes over the solid angle occupied by clutter. Being non-adaptive, this process computes element or subarray perturbations based on the simulated or measured amplitude and phase responses of the subarray or element channels, including the angle dependent subarray or element gain patterns that are unique to each channel. As simulated or measured channel responses differ from the actual channel responses by unknown amplitude and phase errors, the nulling weights computed therefrom are erroneous, resulting in nulls of unduly limited depth. Residual unknown amplitude and phase errors remain, even after antenna calibration. Further, a non-adaptive approach positions and shapes the nulls without regard for the actual angle distribution of the clutter. This results in a mismatch between the two-way null shape obtained and the clutter's true angle distribution. Such mismatches result even when the transmit and receive element or subarray channels are substantially error free.
Alternative systems and methods for cancelling ground-based radar sidelobe clutter are desired.